Le Mans Racer's Mystery Bumps Explained

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Le Mans Racer's Mystery Bumps Explained

This weekend is the annual gathering of sports cars in Le Mans, France for the most prestigious endurance car race of the year, the 24 Hours of Le Mans. Raced over a mix of public roads and race track, the eight and a half mile road course is a challenge to engineers with much of the 24 hour race spent at wide open throttle, but with a mix of tight corners where down force is needed for better cornering.

Each year in most forms of auto racing there is a battle between those who create the rules governing the cars, and the engineers who want to make their cars as fast as possible during the race.

So when the Oak Racing team showed up for early testing at Le Mans with some mysterious bumps on the rear wing of their prototype racer, just about everybody started scratching their heads wondering what the team was trying to accomplish. At first glance they look like some sort of aerodynamic device to reduce drag, but it turns out the bumps are only indirectly responsible for drag reduction.

Sports car racing journalist Mike Fuller went searching for an answer, and what he found was a rather creative and eloquent effort to reduce drag within the rules that dictate the shape of the rear wing.

The governing body for the cars that race at Le Mans dictate that there must be a 20 millimeter tab on the rear edge of the wing on this class of race cars. This is presumably to keep speeds in check by adding some drag. The rule stipulates that this tab must be at 90 degrees to the imaginary line created from the top of the wing to the rear edge of the wing.

As Fuller diagrams on his website (bottom of the page), by adding the alternating bumps to the top of the wing, Oak Racing has effectively raised the top of the wing and in turn allowed the 90 degree tab to rotate back, reducing the aerodynamic force on the tab, and in turn reducing the drag. But because the bumps are on the top of the wing, they do not interfere dramatically with the down force being created largely by the shape of the bottom of the wing. It's similar to how the bombs and fuel tanks hanging from the wing of a fighter jet don't dramatically affect the wing's ability to create sufficient lift. But in the case of the car, the wing is upside down.

The race gets underway tomorrow at 3pm in Le Mans (9am Eastern). The team over at Racecar Engineering has put together a great guide to this year's race with plenty of technical details on all of the cars.